In electrophotography, a latent image is created on the surface of an insulating, photoconducting material by selectively exposing an area of the surface to light. A difference in electrostatic charge density is created between the areas on the surface exposed and those unexposed to the light. The latent electrostatic image is developed into a visible image by electrostatic toners containing pigment components and thermoplastic components. The toners, which may be liquids or powders, are selectively attracted to the photoconductor's surface, either exposed or unexposed to light, depending upon the relative electrostatic charges on the photoconductor's surface, development electrode, and the toner. The photoconductor may be either positively or negatively charged, and the toner system similarly may contain negatively or positively charged particles.
A sheet of paper or intermediate transfer medium is given an electrostatic charge opposite that of the toner and then passed close to the photoconductor's surface, pulling the toner from the photoconductor surface onto the paper or intermediate medium still in the pattern of the image developed from the photoconductor surface. A set of fuser rollers or belts, under heat, melts and fixes the toner in the paper, subsequent to direct transfer or indirect transfer when an intermediate transfer medium is used, producing the printed image.
The electrostatic printing process, therefore, comprises an ongoing series of steps in which the photoconductor surface is charged and discharged as the printing takes place. In addition, during the process, various charges are formed on the photoconductor surface, the toner and the paper surface to enable the printing process to take place. Having the appropriate charges in the appropriate places at the appropriate times is what makes the process work.
Contamination of print media arises in electrophotographic printers and copiers as a result of charge accumulation on the fuser hot roll or belt and the pressure roll. This contamination results from the offset of toner from the print media onto the contacting fuser hot roll or belt due to unfavorable electrostatic fields in and around the fusing nip (i.e., the nip formed between the fuser roll or belt and the pressure roll). This contamination ("toner offset") results in a printed page of poor quality, generally characterized by the appearance of undesired white lines followed by toner debris after one additional revolution of the fuser hot roll or belt.
Prior solutions to this problem focus on controlling the resistance of the coating on the fuser hot roll or belt in combination with underlying electrodes which may be grounded or tied to a bias source. Using such an approach, a fuser hot roll has a conductive (typically metal) core with one or more fluoropolymer coatings which may be loaded with electrically conductive particles in addition to thermally conductive or reinforcing particles. Similarly, a fuser film belt would have a high tensile modulus substrate layer (typically a polyimide layer) loaded with thermally conductive particles (typically boron nitride), a conductive primer layer (e.g., carbon black loaded fluoropolymer), and an outer layer which is made resistive by the addition of conductive particles (such as carbon black) or ionic conductive additives to a fluoropolymer resin. In an alternative approach, the pressure roll may be comprised of materials which limit build-up of surface charge and make it usable as an electrode. Using this approach, a metal core or shaft would be covered with a compressible rubber material that is loaded with carbon black to make it resistive. A fluoropolymer is applied to form a surface layer on the pressure roll which is rendered resistive by the addition of carbon black or an ionic conductive agent. The resistive nature of these coatings bleeds off the surface charge. Examples of this approach are described in the patents cited below. The problem with this approach is that it requires particulate materials, such as carbon black, in each of the layers on the fuser roll or belt, or pressure roll, particularly in the outer layer (i.e., the layer which comes in contact with the printed page), which renders release of the printed page from the fuser more difficult.
Japanese Laid Open Application 7-199700, Canon K. K., filed December, 1993, describes a fusing belt for use in an electrophotographic process which is said to prevent charge accumulation on the belt. The belt comprises an insulating substrate, a conductive primer layer, and a high resistance release layer, such as the fluororesin PTFE with silica particles dispersed in it.
U.S. Pat. No. 4,179,601, Tarumi, et. al., issued Dec. 18, 1979, describes a fixing (fusing) apparatus for an electrophotographic process which reduces the level of electric charge on the fixing roll surface. The fixing roll and/or press roll in this device is taught to have an outer layer comprised of a resinous material with a low electric resistance powder incorporated therein (such as the fluororesin PTFE having carbon black and titanium dioxide incorporated therein).
U.S. Pat. No. 4,434,355, Inigaki, et. al., issued Feb. 28, 1984, describes a heat fixing device for use in an electrophotographic process which is said to inhibit toner offset. The heat fixing roll described includes an outer layer comprised of a fluororesin (such as PTFE, PFA or FEP) containing from 9% to 25% of carbon fibers.
U.S. Pat. No. 4,550,243, Inagaki, issued Oct. 29, 1985, also describes a heat roll fixing device for use in an electrophotographic process which is taught to inhibit toner offset. The roller comprises an electrically conductive core which carries a primer layer containing particulate carbon black with a fluororesin layer on top of it; the primer layer is partially exposed at the surface of fluororesin layer. The charges produced on the surface of the fluororesin layer are released by grounding through the primer layer and the conductive core. See also U.S. Pat. No. 4,596,920, Inagaki, issued Jun. 24, 1986.
U.S. Pat. No. 5,045,891, Semba, et. al., issued Sep. 23, 1991, describes an image fixing apparatus which is said to inhibit toner offset. The heating roll comprises an electrically conductive core which carries a fluororesin layer (such as PFA or PTFE) which includes 3% to 20% of a low resistance single crystal fiber, such as potassium titanate, silicon carbide, or carbon. These fibers are said to form conductive paths from the surface of the roll to the conductive core which acts to discharge any surface charge formed.
Until now, the electrical breakdown characteristics of the fuser roll or belt has not been used as a primary criterion for formulating a roll or belt which minimizes toner offset. It has now been found that if a fuser roll or belt is formulated such that it exhibits electrical breakdown at 250 volts or less, the toner offset contamination problem associated with charge accumulation on the fuser belt or roll is eliminated. Further, it is possible to formulate such a roll or belt with no particulate material in the outer layer thereby improving the release characteristics in the printing process.